I was learning about the physics behind "mirascopes" from a number of online resources. It's explained that a mirascope consist of two concave parabolic mirrors of equal focal length placed on top of each other in such a way their principal axes coincide. Further the distance between the poles of the two mirrors is exactly equal to the focal lengths of the two mirrors.
Let us consider the following diagram:
An object placed on the bottom mirror will be at the focal plane of the mirror at the top. Hence, rays emerging from the object will get reflected parallel to the principal axis after the reflection from the upper mirror. These rays being parallel to the principal axis get focussed at the focal plane of the mirror at the bottom after second reflection. Thus a real inverted image is formed near the hole.
On the lines of my understanding, either parabolic or spherical concave mirrors could be used since their functionality is almost same for an object placed at their focal points. However, according to the sources I referred, only parabolic mirrors are used to make mirascopes. So, what happens if we use a spherical concave mirror instead of a parabolic mirror? What will be the effects on the image formed?
I think a parabolic mirror is a better option over concave mirror, as the spherical aberration in the former is comparatively lesser than the latter one. I inferred this point from the following text from a Wikipedia article:
Spherical mirrors, however, suffer from spherical aberration — parallel rays reflected from such mirrors do not focus to a single point. For parallel rays, such as those coming from a very distant object, a parabolic reflector can do a better job.
But I'm not sure whether this is the reason why parabolic mirrors are used in mirascopes as I think for a object which doesn't produce it's own light, it's illuminated only because of light coming from the hole on the upper mirror and hence marginal rays (rays far away from principal axis) are not of great importance.
Image Courtesy : My own work :)